/Engine/Source/Runtime/Core/Public/Misc/CoreDelegates.h
Called when in the background, if the OS is giving CPU time to the device. It is very likely this will never be called due to mobile OS backgrounded CPU restrictions. But if, for instance, VOIP is active on iOS, the will be getting called
static [TMulticastDelegate](API\Runtime\Core\Delegates\TMulticastDelegate)< void(float DeltaTime)> MobileBackgroundTickDelegate;